This invention relates to polycarbonate mouldings having improved optical properties.
Polycarbonate mouldings, in particular polycarbonate solid sheets, are produced by extrusion or by injection moulding. Larger mouldings, such as large areas of glazing for motor vehicles, have to be produced by extrusion, because production by injection moulding can be carried out only under high pressures or mould-locking pressures which are not economically justifiable.
For the extrusion process, a polycarbonate granular material is fed into the extruder and melted in the plasticising system of the extruder. The plastics melt is pressed through a slot die, brought into the required shape in the roll nip of a glazing calender and set by alternate cooling on smoothing rolls and in the ambient atmosphere. The polycarbonates having a high melting viscosity which are used for the extrusion are conventionally processed at melting temperatures of 260 to 300xc2x0 C.; the cylinder temperatures of the plasticising cylinder and the die temperatures are adjusted appropriately.
The surfaces of the sheet produced as a result of this process are not completely flat and exhibit irregularities. These give rise to optical distortions. Such optical distortions are undesirable for various applications, for example, glazing in motor vehicles, for which reason a number of measures for improving the optical properties of extruded polycarbonate mouldings been already been proposed in the past.
To decrease reflections and optical interference in the case of structured, weather-resistant sheets, EP 0 275 252 A proposes a polycarbonate laminated panel, to the surface of which is applied a protective coating which contains a methacrylate copolymer.
EP 0 114 290 A describes thermoplastic copolyester carbonate moulding compositions containing 1 to 15 wt. % of a branched polycarbonate and 15 to 99 wt. % of a copolyester carbonate resin. The processing properties and the resistance to hydrolysis as well as the impact strength are said to be improved by this mixture. It is found that the optical transparency of the polymer is not fundamentally impaired by the use of this moulding composition.
The object of the invention is to provide transparent polycarbonate solid sheets produced by extrusion and having optical properties improved to the extent that they permit use in motor vehicles.